Disposal of residual waste water from industrial plants may pose difficult and costly problems. Many chemical manufacturing plants have on-site facilities to treat their waste waters in order to ensure that potential pollutants in the treated waste water are reduced to levels required to comply with local and/or national regulations for disposal of waste water into local sewage treatment systems, rivers, lakes or oceans.
Industrial sites, such as abattoirs, food manufacturing plants, polymer recycling plants and the like, produce large quantities of waste liquids, typically in the form of aqueous dispersions, including particulate matter of various types, such as solid particles as well as particles in the form of liquids such as fatty particles or oil droplets.
Although such sites may be capable of direct connection to the sewage system, local authorities in control of the sewage system may apply limits to the purity or level of contamination of waste water which can enter the sewage system.
In particular, some authorities may impose penalties or fines upon sites when excessively contaminated waste water is allowed to enter the sewage treatment system, or the environment, from the site.
However, such sites may produce waste streams with wide variability in the contaminant levels present. For instance, the concentrations of contaminants present in the waste stream from such a plant may be considerably lower during wash-down and cleaning than they are during normal production operation of the plant.
It is desirable to provide an on-site waste stream treatment plant that is capable of dealing with purification of waste streams having high variability in contaminant content in an efficient manner.
Furthermore, it is desirable to avoid waste water, whilst awaiting treatment, being stored within a manufacturing plant, as this may lead to risk of contamination to the manufactured goods from the stored waste water. Similarly, it is desirable that waste products separated from the waste water are not stored or retained in proximity with the manufacturing plant or manufactured goods.
Hence, there is a need for methods and plant for the efficient, stand-alone treatment of waste liquids from such industrial sites so that water recovered from the waste liquids may be returned to the environment, such as through the local sewage system.
For coarse particles and liquid fats, physical separation by classification and/or skimming is effective, but this may result in a remaining aqueous dispersion of particulate matter which may be difficult to separate from water in an efficient manner. Typically, the particles may be in a colloidal state, in other words having a particle diameter from about 1 to 10,000 nm. Colloidal dispersions may be difficult to separate fully and efficiently.
The stabilisation and aggregation of colloidal dispersions or emulsions of particles in water or in aqueous solutions, has been explained in terms of DLVO theory (an acronym for the workers Derjaguin, Landau, Verwey and Overbeek who developed the theory) which combines the effects of van der Waals attraction with electrical double layer repulsion between dispersed, charged colloidal particles.
Commonly charged colloidal particles (i.e. colloidal particles having the same sign of charge) are stabilised in colloidal dispersions by mutual electrostatic repulsion forces exceeding the attractive van der Waals attraction.
The charged particles may attract counterions, of opposite charge to their charged surfaces, from their aqueous surroundings, resulting in the formation of an electrical double layer (EDL) at the particle surface. This EDL screens the electrical repulsion between particles, and so by formation of a suitable EDL, the electrostatic repulsion between the commonly charged colloidal particles may be sufficiently screened in order to allow van der Waals forces to drive coalescence of the particles into larger, bulk agglomerates or flocs.
For water purification, or for extraction of desired materials from an aqueous dispersion or slurry, in order to remove colloidal particles from water by flocculation, modification of the EDL may be achieved by addition of electrolyte to the colloidal dispersion to be flocculated. However, for water purification, this has the disadvantage that high levels of dissolved electrolyte may remain in the water remaining after flocculated material has been removed.
Electrocoagulation is based upon the use of electrochemical dissolution of an electrode by electrolytic oxidation with OH− to form counterions of high charge, at the anodes, which can aid flocculation (typically cations such as Fe3+ or Al3+ for flocculation of fatty particles) without the need for addition of corresponding salt-derived anions into the liquid to be treated (typically OH− will be the counterions formed in the electrocoagulation process). In parallel with the formation of the cations formed at the anode, gas bubbles (hydrogen) are also formed at the cathode. The term “electrocoagulation” as used herein is also meant to encompass electroprecipitation.
For a typical electrocoagulation system, opposed electrodes may be used to provide a voltage difference across one or more sacrificial electrodes positioned between the opposed electrodes, with the sacrificial electrodes not electrically connected to each other or to the opposed electrodes other than through the liquid being treated. This results in an electrical field being set up across the sacrificial electrodes, causing them to have cathodic and anodic surfaces and causing a current to flow between them and the opposed electrodes, typically with the material of the sacrificial electrodes oxidising and dissolving at the anodic surfaces and hydrogen bubbles being generated at the cathodic surfaces. For instance with sacrificial electrodes of aluminium, aluminium hydroxide is formed at the cathode and can lead to flocculation or co-precipitation of colloidal particles within the liquid to be treated. Typically, a voltage of 50 to 600V may be applied, with a direct current, such as up to 60 A, for instance from 1 to 55 A, say from 5 to 20 A passing between the opposed electrodes.
A problem with electrocoagulation systems is that the aqueous dispersion passing through the electrodes may cause the electrodes to become subject to excessive contamination or coating, which may give rise to the need to replace the electrodes at intervals as they become coated by contaminant during use.
In general, because of the problems associated with such contamination build-up, electrocoagulation has not been seen as a suitable method for the generation of purified water from industrial waste streams containing fat particles, such as those from abattoirs or food manufacturing plants, particularly when the fat and/or electrolyte content of the waste stream is subject to high variability over time.